[Merged by Bors] - feat: port FieldTheory.Adjoin

Mathlib.lean

@@ -1612,6 +1612,7 @@ import Mathlib.Dynamics.Flow
 import Mathlib.Dynamics.Minimal
 import Mathlib.Dynamics.OmegaLimit
 import Mathlib.Dynamics.PeriodicPts
+import Mathlib.FieldTheory.Adjoin
 import Mathlib.FieldTheory.AxGrothendieck
 import Mathlib.FieldTheory.ChevalleyWarning
 import Mathlib.FieldTheory.Finite.Basic